You could hang your own pressurized storage tank on your line with a check valve preventing backflow. However that would only give you a small amount of standby water.
An unpressurized storage tank followed by a pressure pump and pressure tank and with a protective check valve on the input from the well would create your own water system and you would only be limited by how big your storage tank is. You would still need to know if water from the well becomes unavailable or you tank will run dry eventually.
How much water can the well produce continuously? If a faucet is left open, or perhaps more likely, a pipe fails, will the well be able to supply this constant load and other home's needs? If so the problem becomes one of excessive power use and accelerated pump wear.
A simple, but unreliable, solution is to have everyone shut off the water feed for their house whenever they are not staying there.
Depending on circumstances you may want to work out a way to monitor the well. The simplest, and berhaps least useful, would be to bury some wires with the water pipe and build some simple electronics tied to the pump motor circuit to show when the motor is running. This should be done in such a way that the monitor voltage is something low (e.g., 12 Volts). You could do this with a small power supply controlled by a realy coil in parallel with the pump motor (obviously matched to motor voltage). You can run a wire to each house. I would place some resistance in the power supply output to limit current flow if someone does something stupid to the wire. A small box with an LED and a suitable dropping resistor finishes the system.
Now this requires someone noticing what is going on and deciding wheather it should be running that much or not. A small compatible run time meter would be useful if you keep track of things.
From there, if you get a small controller it could be programmed to watch pump runtime and light an alert light in the 3 houses if it runs more than the programmed limit in some period of time. Get really clever and you can log all the operation and look at a complete usage history.
You could also put in water meters in each of the lines. You can get ones that provide an electrical output (relay closure) per unit of water used. Some provide a remote digital readout. This would let you apportion billing to users and if all three indications are brought to each house the realtime use of water could be observed. Electronics could be used to log, alarm, etc. limited only by skill and imagination.
Pressure monitoring may also provide desired information depending on the well production characteristics.